This project investigates the joint consequences of bilingualism and Alzheimer's disease (AD) for linguistic and cognitive processing, with the goals of shaping psycholinguistic models of bilingualism (Aim 1), testing models of cognitive decline in AD (Aim 2), and characterizing the profile of cognitive impairments typical of bilingual AD (Aim 3). Data from an initial 5 years of funding revealed unexpected patterns of dual-language decline in AD, and suggested that bilinguals perform differently from monolinguals on neuropsychological tests often used to diagnose AD. These results challenge models of bilingualism and cognitive decline in AD, which currently do not account for patterns of cognitive change in bilingual AD, and they underscore the need to further investigate how factors that are known to affect bilingual language processing influence the presentation of AD in bilinguals. We consider three major accounts of bilingual language processing to derive predictions about dual-language decline in AD, while also aiming to identify which language, which tasks, and which testing conditions produce the greatest differences between Spanish-English bilinguals with AD and matched cognitively healthy controls. As a part of this endeavor, we propose to extend our investigation of language deficits in bilingual AD from single word production to also examine production of connected speech and verbal list memory. The proposed studies provide a unique test of models of cognitive decline in AD, which, when considered in concert with different bilingual processing models, lead to different predictions about dual-language change in bilingual AD. By testing these models, we aim to identify which cognitive mechanism is responsible for linguistic decline in bilingual AD (semantic processing, retrieval difficulty, or executive control), to shed light on the role of executive control in bilingual language processing and, possibly, to identify protective effects of bilingualism against cognitive decline, and unique markers of AD in bilinguals. Using objective measures of bilingual language proficiency, each study will examine how bilingualism affects performance across the full continuum from low to high levels of proficiency. The proposed studies will continue a systematic program of research that aims to characterize the typical presentation of cognitive deficits in bilingual AD, while bringing unique evidence to bear on models of bilingual language processing and cognitive decline in AD. The proposed studies will provide a theoretical framework from which to develop better assessment measures for bilinguals, and they will have further practical implications by identifying conditions that minimize interference between languages and allow bilinguals with AD to speak as fluently as they can.